1. Field of the Invention
The present invention relates to a receiving apparatus in which a plurality of antennas are provided and an antenna in an optimum reception state is selected, and also relates to a signal processing circuit used in the receiving apparatus, and a signal receiving method.
2. Description of the Related Art
Conventionally, in a TV tuner and radio tuner for vehicle, a diversity receiver provided with a plurality of antennas is used for the purpose of receiving the best radio waves at all times because the state of received radio waves changes continually as the vehicle moves, and the antennas are switched in a case where a level of multipath noise included in a reception signal exceeds a threshold value.
In order to select an antenna in a good radio wave condition without executing a complicated switch-over when switching antennas in such a diversity receiver, a sensitivity varying system is adopted.
To be more specific, in a case where antennas A and B are in such a noise occurrence condition that the multipath noise level is high as illustrated in FIGS. 10A and 10B, switch-over between the antennas A and B is performed every time the noise level exceeds antenna switching sensitivity (a threshold value) Vth as illustrated in FIG. 10C, and every time the switching is performed, the antenna switching sensitivity is decreased (the threshold value is increased) gradually depending on the noise level and so on (for example, refer to Japanese Unexamined Patent Publication JP-A 2003-60539).
That is to say, since such a condition that the antennas are switched continually is brought when the level of the threshold value Vth is too low with respect to a noise component, that is, the noise detection sensitivity of a comparator is too high, the noise detection sensitivity of the comparator is decreased to proper sensitivity by increasing the threshold value Vth, whereby an antenna switching controlling section is prevented from frequently performing antenna switch-over.
FIG. 11 is a view illustrating the configuration of a conventional diversity receiver. In this diversity receiver, an antenna switching circuit 21 performs switch-over to select one of high-frequency reception signals generated when antennas ANT1 and ANT2 receive coming radio waves, and a high-frequency reception signal from one of the antennas selected by switch-over is inputted to an intermediate-frequency amplifying circuit 23 via a front end 22.
And then, an intermediate-frequency signal outputted from the intermediate-frequency amplifying circuit 23 is supplied to an outputting circuit (not illustrated) provided with an audio amplifier and so on, as well as supplied to an S meter circuit 24 that detects electric field intensity, and an electric field intensity signal from the S meter circuit 24 is inputted to a noise detecting filter 25, where a noise component is detected.
This noise component is inputted to a comparing circuit 27 via a noise AGC circuit 26 and compared with the threshold value Vth. When the output of the noise AGC circuit 26 gets larger than the threshold value Vth, the comparing circuit 27 outputs a signal to a selection signal outputting circuit 28, and the selection signal outputting circuit 28 causes the antenna switching circuit 21 to switch the antennas. When the frequency of detection of noise gets high, the noise AGC circuit 26 decreases gain to decrease the noise detection sensitivity of the comparing circuit 27 to proper sensitivity, thereby controlling so that antenna switch-over is not frequently performed.
As described above, in the conventional diversity receiver, a noise component included in airwaves received by a selected antenna is detected, and it is determined whether a reception state is good or bad based on the result of detection of noise, with the result that the antennas may be switched. In the case of switching all the antennas because the reception state is bad, the noise detection sensitivity is decreased to more proper sensitivity in order to prevent excessive antenna switch-over.
However, in such a system that the sensitivity is gradually decreased, the antenna switching sensitivity is varied by, for example, time constant of the AGC circuit, so that needless antenna switch-over is performed, and an antenna in a good reception state cannot be selected instantly.